学步园

1. 概念

异常说明/规范(exception specification)指定, 如果函数抛出异常, 被抛出的异常将是包含在该说明中的一种, 或者是从列出的异常中派生的类型.

2. 定义

异常说明跟在函数形参表之后. 一个异常说明在关键字throw之后跟着一个(可能为空的)由圆括号括起来的异常类型列表. 如:

void foo(int) throw (std::logic_error, std::runtime_error);

这个声明指出, foo是接受int值的函数, 返回void. 如果foo抛出一个异常, 该异常将是std::logic_error或std::runtime_error对象, 或者由std::logic_error或std::runtime_error派生的类型的异常.

3. 说明

• 编译时, 编译器不能也不会试图验证异常说明;
• 空说明列表指出函数不抛出任何异常;
• 如果一个函数声明没有指定异常说明, 则该函数可以抛出任意类型的异常;
• 异常说明是函数接口的一部分, 函数定义以及该函数的任意声明必须具有相同的异常说明;
• 异常说明不能用于函数重载;
• 在const成员函数声明中, 异常说明跟在const限定符之后;
• 如果函数抛出了没有在其异常说明中列出的异常, 就调用标准库函数unexpected. 默认情况下, unexpected函数调用terminate函数, terminate函数一般会终止程序;
• 异常说明/规范中不仅要包含函数本身引发的异常, 还应该包含该函数调用的其它函数引发的异常, 以此类推;
• 引发异常时, 编译器总是创建一个临时拷贝, 即使catch块中指定的是引用(这里指定引用的目的并不是避免创建拷贝以提高效率, 而是: 基类引用可以执行派生类对象.). 如: 
  class problem {...};<br /> ...<br /> void super() throw (problem)<br /> {<br /> ...<br /> if (oh_no)<br /> {<br /> problem oops; // construct object<br /> throw oops; // throw it<br /> }<br /> ...<br /> }<br /> ...<br /> try<br /> {<br /> super();<br /> }<br /> catch (problem& p)<br /> {<br /> // statements<br /> }<br /> // 说明: p将指向oops的拷贝而不是oops本身. 这是件好事, 因为函数super()执行完毕后, oops将不复存在. 不过这里可能更简单:<br /> // throw problem();  

4. 代码

// usr_excep.h -- exception classes for hmean(), gmean()</p> <p>#include <iostream> </p> <p>class bad_hmean<br /> {<br /> private:<br /> double v1;<br /> double v2;<br /> public:<br /> bad_hmean(double a = 0, double b = 0) : v1(a), v2(b){}<br /> void mesg();<br /> };</p> <p>inline void bad_hmean::mesg()<br /> {<br /> std::cout << "hmean(" << v1 << ", " << v2 <<"): "<br /> << "invalid arguments: a = -b/n";<br /> }</p> <p>class bad_gmean<br /> {<br /> public:<br /> double v1;<br /> double v2;<br /> bad_gmean(double a = 0, double b = 0) : v1(a), v2(b){}<br /> const char * mesg();<br /> };</p> <p>inline const char * bad_gmean::mesg()<br /> {<br /> return "gmean() arguments should be >= 0/n";<br /> }

//foo.cpp -- unwinding the stack</p> <p>#include <iostream><br /> #include <cmath> // or math.h, unix users may need -lm flag<br /> #include <cstring><br /> #include "usr_excep.h"</p> <p>class demo<br /> {<br /> private:<br /> char word[40];<br /> public:<br /> demo (const char * str)<br /> {<br /> std::strcpy(word, str);<br /> std::cout << "demo " << word << " created/n";<br /> }<br /> ~demo()<br /> {<br /> std::cout << "demo " << word << " destroyed/n";<br /> }<br /> void show() const<br /> {<br /> std::cout << "demo " << word << " lives!/n";<br /> }<br /> }; </p> <p>// function prototypes<br /> double hmean(double a, double b) throw(bad_hmean);<br /> double gmean(double a, double b) throw(bad_gmean);<br /> double means(double a, double b) throw(bad_hmean, bad_gmean);</p> <p>int main()<br /> {<br /> using std::cout;<br /> using std::cin;<br /> using std::endl;</p> <p> double x, y, z;<br /> demo d1("found in main()");<br /> cout << "Enter two numbers: ";<br /> while (cin >> x >> y)<br /> {<br /> try<br /> { // start of try block<br /> z = means(x,y);<br /> cout << "The mean mean of " << x << " and " << y<br /> << " is " << z << endl;<br /> cout << "Enter next pair: ";<br /> } // end of try block<br /> catch (bad_hmean & bg) // start of catch block<br /> {<br /> bg.mesg();<br /> cout << "Try again./n";<br /> continue;<br /> }<br /> catch (bad_gmean & hg)<br /> {<br /> cout << hg.mesg();<br /> cout << "Values used: " << hg.v1 << ", "<br /> << hg.v2 << endl;<br /> cout << "Sorry, you don't get to play any more./n";<br /> break;<br /> } // end of catch block<br /> }<br /> d1.show();<br /> cout << "Bye!/n";</p> <p> return 0;<br /> }</p> <p>double hmean(double a, double b) throw(bad_hmean)<br /> {<br /> if (a == -b)<br /> {<br /> throw bad_hmean(a,b);<br /> }</p> <p> return 2.0 * a * b / (a + b);<br /> }</p> <p>double gmean(double a, double b) throw(bad_gmean)<br /> {<br /> if (a < 0 || b < 0)<br /> {<br /> throw bad_gmean(a, b);<br /> }</p> <p> return std::sqrt(a * b);<br /> }</p> <p>double means(double a, double b) throw(bad_hmean, bad_gmean)<br /> {<br /> double am, hm, gm;<br /> demo d2("found in means()");<br /> am = (a + b) / 2.0; // arithmetic mean<br /> try<br /> {<br /> hm = hmean(a, b);<br /> gm = gmean(a, b);<br /> }<br /> catch (bad_hmean & bg) // start of catch block<br /> {<br /> bg.mesg();<br /> std::cout << "Caught in means()/n";<br /> throw; // rethrows the exception<br /> }<br /> d2.show();</p> <p> return (am + hm + gm) / 3.0;<br /> }